This invention relates to an apparatus for recovering sulfur from gases containing hydrogen sulfide by the process described in German Auslegeschrift No. 1,941,703, which apparatus is, however, suitable for processing gas mixtures containing any concentration of hydrogen sulfide met with in industrial processes.
When industrial gases containing more than 15 percent by volume of hydrogen sulfide are produced and are thus intended for further treatment, it has hitherto been generally customary to oxidize with atmospheric oxygen the appropriate quantity of hydrogen sulfide present in the feed gas in order to produce the quantity of sulfur dioxide required by the proportions for the recovery of sulfur. The appropriate quantity of air for combustion is precisely adjusted and continuously regulated automatically in accordance with the quantity and composition of the gas containing hydrogen sulfide. The reaction is carried out at high temperatures (approximately 1,000.degree. C.) in an open flame in a bricklined combustion chamber. The mixing of the combustion gas with the residual gas takes place simultaneously in this combustion chamber in the absence of any operational or external action on the mixing process.
The following text summarizes the effects which take place, or are to be expected, in accordance with experience, on the overall reaction and thus on the overall yield when a sulfur recovery plant is operated in the above-mentioned manner.
As a result of the oxidation of hydrogen sulfide with air, water of combustion is formed simultaneously and in a quantity equal to that of the sulfur dioxide required for the reaction. Since the reactions leading to the formation of sulfur are reversible, the effect of increasing the proportion of water in the reaction gas mixture is to cause a shift to the left in the equilibrium in the equation: 2H.sub.2 S+SO.sub.2 .revreaction.2H.sub.2 O+3S; this shift in the equilibrium is associated with a corresponding reduction in the yield of sulfur. At the same time, the substantially greater water content of the gas means a considerable increase in the dew point. In order to avoid corrosive effects caused by condensation, it is necessary to keep the operating temperatures in all sections of the plant carrying gas always at a temperature which has an adequate safety margin above the dew point temperature. The most advantageous operating temperature in the catalytic reaction of the sulfur is within a relatively low temperature range. As soon as this temperature range is exceeded, the formation of sulfur is necessarily reduced, regardless of the cause. In order to keep the yield constant, it would either be necessary to reduce the space velocity in the catalyst containers, or alternatively the throughput of the sulfur recovery plant, or to increase the quantity of catalyst.
Stemming from the raw materials, for example petroleum and coal, proportions of hydrocarbons, carbon oxysulfide and carbon disulfide, which in most cases also vary in quantity and composition, are present in the mixed gases containing hydrogen sulfide, which are obtained from desulfurization plants for further processing in sulfur recovery plants. The effect of the non-catalytic high temperature--combustion at about 1,000.degree. C. to 1,300.degree. C.--is to promote, in the combustion chamber, not only the dissociation into its elements of hydrogen sulfide, but also, in particular, that of the hydrocarbon. The formation of H.sub.2 O and CO.sub.2 in conjunction with the free atmospheric oxygen is only brought about to a limited extent. The moderated and greatly retarded formation of SO.sub.2 can also be explained by the high temperatures in the radiant zone of the free flame. The cracked products from the hydrocarbon combine principally with the diatomic sulfur which is present in abundant quantities in the form of vapor and is considerably superheated. Admittedly these compounds can, to a limited extent, be re-decomposed and converted into elementary sulfur in one of the down-stream catalytic reaction stages. However, this requires a very highly active catalyst and, in addition, an operating temperature which is far above the most favorable figures which have been determined for the reaction of H.sub.2 S with SO.sub.2. A diminution in this reaction is a fundamental factor for the overall yield and, looking further, determines the quantity of noxious material with which the environment will be polluted in a particular case.
As a result of the process, the oxidation temperature in the combustion chamber also rises if there is a greater quantity of hydrogen sulfide in the feed gas. If the combustion is carried out with air, it must in any case be expected that the formation of nitric oxide will be promoted at very high temperatures. For this reason, a detectable quantity of nitric oxide can, in some cases, be present in the reaction gas if the operating procedures which are customary according to the state of the art known at the present time are used. It is assumed that oxides of nitrogen are a possible cause of certain obstructions which are often found in the layers of catalyst after interruptions to operation. Gaseous nitric oxide, which dissolves in the liquid sulfur when the gaseous sulfur condenses in the phase of its formation, results in undesirable impurities in the end product.
Corrosive effects and blockages which are difficult to locate can be initiated in the liquid sulfur lines as the result of the formation of crystals containing nitrogen. The nitric oxide content in the exit gases which pass into the free atmosphere should be monitored precisely. The exit gas rate and the exit gas temperature are decisive factors for the quantity of sulfur which is passed in the form of gas and mist, first to the after-combustion furnace, and from there as SO.sub.2 to a chimney stack. To sum up, the atmospheric nitrogen which is necessarily associated with the use of atmospheric oxygen entails, for the processes hitherto customary, the considerable disadvantages that the conditions required for the development of undesirable side reactions are provided, that the reaction gas mixture is so greatly diluted with inert constituents that the conversion and also the yield undergo an appreciable reaction, and that the losses in the exit gases caused by entrained flying particles of elementary sulfur increase in step with the ratio between the quantity of nitrogen and the total quantity of exit gas.
The facts listed in the above description and also many further unsolved problems and unexplained difficulties originate from carrying out, on an industrial scale, the existing process for recovering sulfur from gases containing hydrogen sulfide. An exact proof on the basis of operating results is not yet possible for many valuable pieces of experience in this field of work. For example, no serviceable method of determination has yet been developed for carrying out systematic sampling and satisfactory determination of the composition of the reaction gas mixture at such high combustion temperatures at the exit of the combustion chamber. In most cases the results obtained by gas analysis are much better and more favorable than the operating figures actually found. Since the reaction of hydrogen sulfide with sulfur dioxide is favored as the temperature falls, the formation of sulfur continues in the sampling container after a sample has been taken. It has been proved by tests that at temperatures between 250.degree. C. and 350.degree. C., the reaction only proceeds to completion on solid surfaces and best on glass, aluminum or Al.sub.2 O.sub.3.
The scope of the facts available from operating experience or from tests carried out, and also of the data available from the specialized literature on the relationships and the actual course of the formation of sulfur, is relatively small.
If the process in German Auslegeschrift No. 1,941,703 is used, it is possible to eliminate completely the disadvantages and difficulties caused by the partial combustion of the hydrogen sulfide with air and, furthermore, to achieve in addition considerable advantages in the construction and in the operation of the plant by feeding in the quantity of sulfur dioxide required for carrying out the process, by means of burning sulfur with oxygen.
At present, a large number of processes and types of equipment are known for the manufacture of sulfur dioxide by burning elementary sulfur with air, oxygen or gases enriched with oxygen, especially for obtaining sulfuric acid. However, jet burners, or alternatively atomization burners or nozzle burners, of a variety of designs are used preferentially.
These types of burner, including their respective accessories for obtaining sulfur dioxide, are unsuitable in every form for a further processing step such as corresponds to the process defined in German Auslegeschrift No. 1,941,703, because the combustion of the liquid sulfur is carried out in every case in the same space in which, at the same time, the gas containing hydrogen sulfide is also added. If the hydrogen sulfide content is 15 percent by volume or less, the quantity of sulfur corresponding to the stoichiometric formation of sulfur dioxide is also reduced proportionately. The heat of combustion liberated is too low to vaporize the sulfur, which is finely atomized in the gas mixture, rapidly and sufficiently into the diatomic state. As a result, it is not possible to expect or to ensure proper and complete oxidation with the very dilute oxygen.